Audacious favourite losing heart (6)
I believe the answer is:
daring
'audacious' is the definition.
(audacious means daring or adventurous)
'favourite losing heart' is the wordplay.
'favourite' becomes 'darling' (darling can mean a favourite).
'losing heart' means to remove the middle letters.
'darling' with its centre removed is 'DARING'.
(Other definitions for daring that I've seen before include "Take (control)" , "Adventurous courage" , "Bold and reckless" , "Venturesome" , "'Adventurous, bold (6)'" .)